1. Field of the Invention
The present invention is directed toward a filter media for a high efficiency particulate air (xe2x80x9cHEPAxe2x80x9d) filter comprising a multiplicity of adjacent electrostatically charged piles of nonwoven fabric. The invention further comprises a method of making a HEPA filter.
2. Description of the Prior Art
HEPA filters have been used to filter radioactive particles out of air streams. HEPA filters are employed in a variety of environments, including hospital operating theaters, electronic manufacturing clean rooms, and nuclear power plants. HEPA filters are employed to remove submicron size particles from the air. The term xe2x80x9cHEPA filter,xe2x80x9d as used herein, refers to a filter that is capable of filtering out at least 99.97% of 0.3 micron size particles, as evidenced by a DOP test.
Fine glass fiber paper has traditionally been used in HEPA filters as well as in ultra low penetration air (xe2x80x9cULPAxe2x80x9d) filters. The term xe2x80x9cULPA filter,xe2x80x9d as used herein, refers to a filter that is capable of filtering out 99.99% of 0.3 micron size particles, as evidenced by a DOP test.
The use of fine glass fiber paper in HEPA filters and in ULPA filters has many disadvantages, including brittleness during processing, heavy weight, high pressure drop, and skin irritation. An additional disadvantage of fine glass fiber paper in this application is that it cannot be electrostatically treated or charged. It has been shown that synthetic charged fibers are well suited as filters in heating, ventilating, and air conditioning (xe2x80x9cHVACxe2x80x9d) systems.
The present invention provides an improved filter media which is capable of meeting the filtration standards of HEPA filters and ULPA filters, while maintaining low pressure drops and low weight. Lower pressure drops result in energy savings. Lower weight results in lower transportation costs. Energy savings and lower transportation costs are advantages of the present invention.
An additional advantage of the present invention is that the material costs are less than 10% the material costs associated with fine glass fiber paper.
The present invention is directed to a particulate air filter media comprising a first pile of charged nonwoven fabric having a weight in the range of 5-40 grams per square meter and one or more fibers having a diameter in the range of 1.5-2.0 microns. The term xe2x80x9ccharged,xe2x80x9d as used herein, refers to electrostatically charging.
The invention also comprises a second pile of nonwoven fabric having a weight in the range of 5-40 grams per square meter and one or more fibers having a diameter in the range of 1.5-2.0 microns, adjacent to said first pile. The invention further comprises a third pile of nonwoven fabric having a weight in the range of 5-40 grams per square meter and one or more fibers having a diameter in the range of 1.5-2.0 microns, adjacent to said second pile.
The present invention is also directed toward a method of making a particulate air filter. This method comprises charging at least three adjacent piles of nonwoven fabric, each of said piles having a weight in the range of 5-40 grams per square meter, and each of said piles comprising one or more fibers having a diameter in the range of 1.5-2.0 microns. This method further comprises collating a stiffening layer with said piles. The method embodiment of this invention further comprises bonding a stiffening layer of fabric to one of said piles to form a stiffened filter media having a top side and a bottom side. The stiffened filter media, comprising the piles of nonwoven fabric and the stiffening layer, is then scored. After scoring, the stiffened filter media is folded into a multiplicity of adjacent layers. The method of the present invention further comprises inserting a separator between adjacent layers.